Abstract

3′-Deoxy-3′-fluorothymidine (FLT) is a positron emission tomography (PET) tracer used to identify proliferating tumor cells. The purpose of this study was to characterize FLT transport by human nucleoside transporters (hNTs) and to determine the role of hNTs for FLT uptake in various human cancer cell lines. FLT binding to hNTs was monitored by the inhibitory effects of FLT on [³H]uridine uptake in yeast cells producing recombinant hNT proteins. hCNT1 displayed the lowest FLT K_i value for inhibition of [³H]uridine uptake, followed by hCNT3, hENT2, hENT1, and hCNT2. [³H]FLT was efficiently transported in Xenopus laevis oocytes individually producing hENT1, hENT2, hCNT1, or hCNT3. [³H]FLT uptake in MCF-7, A549, U251, A498, MIA PaCa-2, and Capan-2 cells was inhibited at least 50% by the hENT1 inhibitor nitrobenzylmercaptopurine ribonucleoside (NBMPR). According to results of real-time polymerase chain reactions, hENT1 and hENT2 had the most abundant hNT transcripts in all cell lines. Cell lines also underwent 1) [³H]NBMPR equilibrium binding assays with or without 5-S-{2-(1-[(fluorescein-5-yl)thioureido]hexanamido)ethyl}-6-N-(4-nitrobenzyl)-5-thioadenosine, a membrane-impermeable NBMPR analog, to determine plasma membrane hENT1 levels, and 2) dose-response NBMPR inhibition of [³H]FLT uptake. MCF-7, A549, and Capan-2 cells displayed NBMPR IC₅₀ values that were smaller or equal to NBMPR K_d values, suggesting that 50% inhibition of hENT1 reduced [³H]FLT uptake by at least 50%. A strong correlation between extracellular NBMPR binding sites/cell and [³H]FLT uptake was observed for all cell lines except MIA PaCa-2. These data suggest that plasma membrane hNTs (especially hENT1) are important determinants of cellular FLT uptake.